Metal is fun. So much fun, in fact, that for many metal musicians the only thing more fun than playing metal is - playing more metal. That's why there are so many metal side projects. Of course, side projects are nothing new and are certainly not confined to metal but - and at the risk of hammering the point home too clumsily - metal is fun! So here are ten of the best. Feel free to share your favorites in the comments below.

Probot

Dave Grohl is a serial collaborator. Aside from his main bands (Nirvana and then Foo Fighters) he's played with Queens Of The Stone Age, Garbage, David Bowie, Nine Inch Nails, Slash, Tenacious D, Paul McCartney, The Prodigy and Them Crooked Vultures, to name but a small sampling. But Dave is a huge fan of metal and the people who make it, so the 2004 self-titled album by his side project Probot sees Dave collaborating with a who's who of metal: King Diamond, Tom G. Warrior, Cronos of Venom, Max Cavalera of Soulfly/Sepultura/Nailbomb/Cavalera Conspiracy, Mike Dean of Corrosion of Conformity, Lee Dorian of Cathedral, Wino of Saint Vitus and The Obsessed, Snake of Voivoid - and of course Lemmy of Motorhead, whose inimitable bass and vocals power the single "Shake Your Blood."

MD.45

When you're the mastermind of one of thrash's Big 4, you pretty much get to do anything you want, at least within the parameters of the genre. And if your name is Dave Mustaine, you get to prod at the edges of that genre with albums like Cryptic Writings and Risk. But Mustaine must have had a slightly more punk sound in mind that he felt he couldn't explore within Megadeth, and it found its outlet in MD.45. The band featured Fear's Lee Ving on vocals, Electric Love Hogs bassist Kelly LeMieux and drummer Jimmy DeGrasso (who would soon end up in Megadeth and is currently playing in Black Star Riders). Their only album, 1996's The Craving, was more raw and definitely more punk than Megadeth. But when Mustaine remastered the album in 2004 he replaced Ving's vocal and harmonica tracks with his own voice and guitar, hoping to attract the interest of Megadeth fans who might not have given the original version a chance.

British Lion

At long last the Iron Maiden bass legend Steve Harris releases a solo album. Just don't expect galloping riffage and gigantic skull-faced sphinxes: British Lion is mainly modern-tinged heavy rock with occasional odd-time prog moments. It's big on melody, and the bass tone is unmistakable even though Harris isn't charging forward in speedy triplets the whole time. Vocalist Richard Taylor's style is far removed from Bruce Dickinson's operatic acrobats but his smoother voice suits the material well. And Harris's bass is right up front, where it belongs.

IR8, Tree of the Sun, Sexurotica

From the time he joined Metallica, Jason Newsted was a solid, reliable cornerstone of the band's sound. Sure, you couldn't really hear him on …And Justice For All, but he was right up front on every subsequent album, and he helped the band to maintain their thrash edge even when they started to drift from that sound. But Newsted's creativity couldn't be contained, and he looked elsewhere for personal fulfillment when not tending to his Metallica duties, including IR8 with Devin Townsend and Exodus's Tom Hunting, Sexurotica with Hunting and Sepultura guitarist Andreas Kisser, and Tree of the Sun, with Townsend, Kyuss's Scott Reeder and The Melvins' Dave Crover. The material by IR8 and Sexurotica was coupled into a 6-track EP released on Newsted's Chophouse Records label in 2002.

A Perfect Circle

A Perfect Circle is one of those bands that falls into two categories at once: side project for some, main band for others. The brainchild of guitarist Billy Howerdel, who spent years on the road as a tech for artists like Nine Inch Nails, Tool, The Smashing Pumpkins, Faith No More and David Bowie, the band's current line-up includes mainstay vocalist Maynard James Keenan of Tool (who has another side project, Pucifer) and former Smashing Pumpkins guitarist James Iha, while previous incarnations have included guitarist Troy Van Leeuwen (Queens of the Stone Age), drummers Tim Alexander (Primus) and Josh Freese (The Vandals, Devo), bass player Jeordie White (Marilyn Manson). With A Perfect Circle spending long stretches of time in hibernation (emerging earlier this year for some festival appearances), Howerdel also releases music under his solo project, Ashes Divide.

Stone Sour

Technically Stone Sour isn't really a side project for vocalist Corey Taylor, since he formed the band long before he joined Slipknot. And Taylor's work ethic is so darn powerful that neither gig could be considered his side band - whatever he's doing, he gives it 100 percent. But since Slipknot was the first of his acts to hit the big time, let's include Stone Sour in this list. There are a few other Slipknot spin-off bands too: drummer Joey Jordison's Murderdolls and his new project Scar the Martyr, and drummer Shawn 'Clown' Crahan's alternative rock band Dirty Little Rabbits.

Stormtroopers of Death

Anthrax's Scott Ian and Charlie Benante teamed up with bass player Danny Liker and singer Billy Milano in this crossover thrash project. Their lyrics were deliberately provocative or as Liker once said, "The lyrics were never intended to be serious, just to [expletive] people off." Their song "March of the S.O.D" was used as the intro music for MTV's Headbanger's Ball for many years.

Praxis

Praxis was an experimental outfit who frequently explored various strains of metal, amid other genres. The band was led by producer/bassist Bill Laswell and featuring a rotating cast of guests but with most incarnations including Buckethead and Bryan "Brain" Mantia. Other guest musicians to pass through the ranks of Praxis over the years included bass legend Bootsy Collins, saxophonist/composer John Zorn, keyboardist Bernie Worrell, Faith No More's Mike Patton, System of a Down's Serj Tankian, and Iggy Pop. Laswell announced in 2011 that Praxis was no more.

Alter Bridge

Alter Bridge is another one of those bands that falls into a grey area that's not quite side project. The band features three quarters of Creed, continuing to work together after that band broke up in 2003. Featuring the vocals of Myles Kennedy - who has also done pretty well for himself as Slash's lead vocalist - Alter Bridge are a little darker and heavier than Creed, with more room for guitarist Mark Tremonti to stretch out. Their fourth album, Fortress, will be released on October 8, 2013 in the USA and on September 30 everywhere else.

Liquid Tension Experiment

Liquid Tension Experiment was a progressive rock supergroup featuring Dream Theater's John Petrucci and then-drummer Mike Portnoy, as well as bass legend Tony Levin and keyboard player Jordan Rudess, who would also end up in Dream Theater before too long. In fact, Dream Theater were courting Rudess to replace original keyboard player Kevin Moore for their Awake album tour in 1994,, but Rudess took a gig with Dixie Dregs instead, not joining Dream Theater until 1999. Liquid Tension Experiment released two studio albums and three lives albums, and they had their own spin-offs, Liquid Trio Experiment and Liquid Trio Experiment 2. The former released the album Spontaneous Combustion in 2007, comprised of jams recorded by Portnoy, Levin and Rudess when Petrucci had to skip out on recording sessions because his wife had gone into premature labor. The latter released a live album called When The Keyboard Breaks: Live in Chicago which documents a 2008 performance in which Rudess spent most of the gig trying to sort out a malfunctioning keyboard, so the remaining members launched into a series of jams and instrument swaps, ultimately ending with Anthrax drummer Charlie Benante playing drums for a jam which included a chunk of Led Zeppelin's "How Many More Times."